Many studies have aimed to produce low-cost silicon feedstock that will be used for growing single-crystal and polycrystalline ingots and for pulling ribbon wafers for manufacturing photovoltaic cells. This “low-cost” feedstock may be obtained by metallurgical purification of the silicon. Metallurgical purification is a purification method in which the silicon never passes into the gaseous phase.
The metallurgically obtained feedstock often contains much greater amounts of dopant impurities (mainly boron and phosphorus) than electronic grade silicon feedstock. In this metallurgically obtained feedstock, the quantities of boron (electron acceptor atom in silicon) and phosphorus (electron donor atom in silicon) are often of the same order of magnitude. “Compensated feedstock” is spoken of.
Photovoltaic cells manufactured using such feedstock often have an energy conversion efficiency that decreases under illumination. This effect is related to the formation, under illumination, of complexes associating a boron atom in a substitutional position (Bs) with an oxygen dimer (O2i). Under illumination the (mobile) oxygen dimer diffuses toward the (immobile) boron atom. The complex formed introduces a deep energy level into the bandgap of the silicon, thereby making recombination of free charge possible, and consequently reducing the lifetime of charge carriers and the energy conversion efficiency of the cell. This degradation has a substantial effect on the conversion efficiency of the cells. Specifically, the efficiency reduction may be about 8 rel. %. This reduction is calculated in the following way: (final efficiency−initial efficiency)/initial efficiency.
Thus, it is important to provide solutions capable of reducing or even preventing the effects of this degradation.